The present invention relates to a light-emitting diode array having a large light-emitting power, particularly to a light-emitting diode array suitably usable for light sources of electrophotographic printers.
An electrophotographic printer forms electrostatic latent image on a photosensitive drum by light corresponding to an image signal, and toner is selectively attached to the electrostatic latent image for development and transferred onto a paper to obtain an image. Widely used as light sources for forming the latent image are of a laser type or a light-emitting diode array type. Particularly light sources constituted by light-emitting diode arrays are suitable for small printers and large-size printing, because they do not need long light paths unlike the laser-type light sources. Recent development of higher-speed, higher-quality printing and miniaturization of printers require light-emitting diode arrays with higher precision and higher output.
Widely known is a light-emitting diode comprising a pair of electrodes on a top surface (light-emitting side) and a rear surface sandwiching a semiconductor substrate and a light-emitting part on the substrate. When voltage is applied to electrodes provided on top and rear surfaces of a light-emitting diode having such a structure, electric current flows in a direction perpendicular to the semiconductor substrate, whereby electrons and holes are recombined in the light-emitting part to emit light. Though there is the maximum light output in the light-emitting part immediately under the top surface electrode, the light generated in this portion is reflected and absorbed by the top surface electrode, failing to be taken out efficiently. Accordingly, as light sources particularly in high-resolution printers of 600 dpi, 1200 dpi, etc., which should have light-emitting portions with small areas, conventional light-emitting diodes are insufficient in light-emitting power.
JP 2000-323750 A discloses a technology of improving the efficiency of taking out the generated light by forming cathodes and an anode on the same surface of a substrate. FIG. 3 is a plan view showing a light-emitting diode array having such a structure; FIG. 4 is a cross-sectional view taken along the line A-Axe2x80x2 in FIG. 3; and FIG. 5 is a cross-sectional view taken along the line B-Bxe2x80x2 in FIG. 3. Incidentally, an insulating layer is omitted to clearly show the underlayers in FIG. 3. A plurality of light-emitting parts 2 are arranged on a p-type GaAs conductive layer 11 formed on an n-type GaAs substrate 10 at a predetermined interval. Each light-emitting part 2 is constituted by a p-type AlGaAs etched stopper layer 12, a p-type AlGaAs clad layer 13, a p-type AlGaAs active layer 14, an n-type AlGaAs clad layer 15 and an n-type GaAs capping layer 16, which are successively laminated on the p-type GaAs conductive layer 11. The light-emitting part 2 has a double hetero structure in a light emission region, which comprises a p-type AlGaAs clad layer 13, a p-type AlGaAs active layer 14 and an n-type AlGaAs clad layer 15.
Each light-emitting part 2 is formed by removing an epitaxial layer by mesa etching. Mesa-etched grooves are constituted by a first mesa-etched groove 21 separating light-emitting parts 2 from bonding portions 8 and second mesa-etched grooves 23 separating light-emitting parts 2.
Part of the top surface of each light-emitting part 2 is provided with a cathode 3. An anode 4 formed in a strip shape on the p-type GaAs conductive layer 11 near the light-emitting parts 2 is a common electrode for operating a plurality of light-emitting parts 2. The cathodes 3 and the anode 4 are formed on the mesa top surfaces of the light-emitting parts 2 and the p-type GaAs conductive layer 11, respectively, by vapor deposition and alloying of metals. The light-emitting parts 2 and the exposed surfaces of the conductive layer 11 except immediately under the cathodes 3 and the anode 4 are covered by an insulating film layer 17 of phosphosilicate glass (PSG). Each Au wiring layer 5 formed with its one end connected to each cathode 3 not covered by the insulating film layer 17 extends to a surface of the bonding portion 8, and the other end of each Au wiring layer 5 is provided with a bonding pad 6.
In the light-emitting diode array with such a structure, an electric current path 19 from the anode 4 to the cathode 3 passes through the light-emitting part 2, resulting in the generation of light L in the p-type AlGaAs active layer 14. This light L is emitted outside from a light-emitting portion 9 provided by removing the n-type GaAs capping layer 16 by etching.
However, in the above conventional light-emitting diode array, as is clear from FIGS. 3 and 4, there is the p-type GaAs conductive layer 11 in the second mesa-etched grooves 23 separating the light-emitting parts 2, there is an electric current path 20 that reaches the cathode 3 from the anode 4 without passing through the p-type AlGaAs active layer 14 in each light-emitting part 2 (FIG. 3). This electric current path 20 may be called xe2x80x9cdetour electric current pass.xe2x80x9d Because electric current passing through the detour electric current path 20 does not contribute to light emission, each light-emitting diode has low light-emitting power.
Accordingly, an object of the present invention is to provide a light-emitting diode array free from a detour electric current path not contributing to light emission, thereby providing increased light-emitting power.
As a result of intensive research in view of the above object, the inventor has found that by removing a conductive layer on a substrate in regions corresponding to second mesa-etched grooves, the above detour electric current path can be eliminated, resulting in increase in the light-emitting power of each light-emitting diode. The present invention has been completed based on this finding.
Thus, the light-emitting diode array of the present invention comprises a conductive layer formed on a substrate, a plurality of separate light-emitting parts formed on the conductive layer, a first electrode formed on at least part of a top surface of each light-emitting part, and a second electrode formed on the conductive layer near the light-emitting part, the second electrode being a common electrode for operating a plurality of the light-emitting parts, and regions of the conductive layer between the adjacent light-emitting parts being removed.
In the light-emitting diode array of the present invention, the light-emitting parts are preferably formed by dividing an epitaxial layer formed on the conductive layer by mesa-etched grooves. The light-emitting diode array according to a preferred embodiment comprises a first mesa-etched groove separating the light-emitting parts from bonding portions for forming a plurality of separate light-emitting parts, and second mesa-etched grooves separating the light-emitting parts, portions of the conductive layer between the light-emitting parts being removed by the second mesa-etched grooves, whereby electric current does not flow between the first and second electrodes without passing through the light-emitting parts.
The mesa-etched grooves preferably are constituted by the first mesa-etched groove separating the light-emitting parts from the bonding portions and the second mesa-etched grooves for removing portions of the conductive layer between the light-emitting parts, in the shape of a comb as a whole.
In the light-emitting diode array of the present invention, electric current does not flow between the first electrodes and the second electrode without passing through the light-emitting parts, resulting in large recombination of electrons and holes in the epitaxial layer in the light-emitting parts and thus increased light-emitting power.